Europe is feeling a bit abandoned these days. Used to regular reassurances from Washington on its commitment to transatlantic security, European leaders are now shifting their defense strategies in response to mixed signals from President Donald Trump on the United States’ role in NATO. Even Secretary of Defense James Mattis has come down hard on the alliance, demanding that Europe “show its support for our common defense” or else Washington would alter the nature of their partnership. Worried Europeans, however, may be slightly comforted by U.S. Secretary of State Rex Tillerson’s decision to attend a NATO foreign ministers’ meeting on March 31, after originally planning to miss the gathering.

Fear that the United States might not live up to its promises, especially to the alliance’s eastern members, has led to a modest uptick in European conventional military outlays, which rose 0.5 percent in 2015 and 3.8 percent in 2016 year in real terms. This is not enough to transform the balance of power, but it is a notable shift. Growing alarm that Europe might end up on its own against Russia has also led to the previously unmentionable idea of developing Europe-wide nuclear deterrence.

Moscow’s 2014 annexation of Crimea from Ukraine and its support for separatists in the Donbass have reminded NATO’s old Europe members that the organization is a military alliance, not an international social club. When NATO extended membership to the Baltic states, few policymakers believed the alliance’s Article 5 promise of mutual security would one day be invoked. Today, they are wondering how they would defend the vulnerable countries without the United States.

Last year, the Atlantic Council’s Matthew Kroenig contended that “NATO must be able to deter a Russian nuclear attack, counter the nuclear coercion inherent in Russia’s hybrid warfare strategy, and assure NATO members that the Alliance is prepared to defend them.” He concluded that this would “require strengthening NATO’s existing nuclear deterrence strategy and capabilities.”